1. Field of the Invention
The invention relates to a communication system which is constructed by an optical fiber line, an optical splitter, and apparatuses arranged at both ends thereof, which enables transmission and reception of a data packet between the apparatuses at both ends, and which enables one of the apparatuses to make management and setting of the other apparatus, wherein a passive optical network (hereinbelow, abbreviated to PON) is used as a network constructed by presuming use of a CMTS (Cable Modem Termination System) and a CM (Cable Modem).
2. Description of the Related Art
In recent years, video distribution services of an IP (Internet Protocol) base mainly according to a video-on-demand form by a communication carrier who provides Internet connection services or a video service providing entrepreneur on the Internet are increasing. Realization of high picture quality of those services is also expected in the future. Therefore, realization of a further high speed of data communication is also demanded in an access line between the communication carrier and general homes or general enterprises as customers (hereinbelow, they are collectively called subscribers).
To realize the high speed of the access line, a construction of the access line using an optical fiber is progressing in each district. Although there are various kinds of systems as communication systems using the optical fibers, the system which has been spread most in the access line is a PON system. The PON system is constructed by the following three kinds of apparatuses: an OLT (Optical Line Terminal) which is provided on a central office (CO) side of the communication carrier and corresponds to a master station; an optical splitter for branch-relaying a photosignal received from one optical fiber toward a plurality of optical fibers as it is; and an ONU (Optical Network Unit; also called ONT (Optical Network Terminal)) which is provided in an individual subscriber's home or a building in which a plurality of subscribers exist and corresponds to a slave station. One optical fiber which connects the OLT and the optical splitter is branched to a plurality of optical fibers by the optical splitter and each optical fiber is connected to one ONU.
As one of forms of the conventional Internet connection services, there is a cable Internet using a coaxial cable line provided for video distribution of a cable television. Although the access line of the cable Internet is generally either a coaxial cable in which the whole interval is a kind of electric wire or an HFC (Hybrid Fiber Coax) in which an interval on the side (upstream side) close to the central office (CO) has been replaced by an optical fiber, the coaxial cable is used in an interval on the side (downstream side) close to the subscriber's home in any of the coaxial cable and the HFC. Generally, the CMTS (Cable Modem Termination System) and the CM (Cable Modem) are used for the apparatus on the central office (CO) side and the apparatus on the subscriber's home side, respectively. The CMTS is an apparatus which has a coaxial cable interface and operates as a router or a layer 2 switch. The CM is an apparatus which has a coaxial cable interface and operates as a bridge or a broadband router.
The cable Internet in which the coaxial cable is used for the access line is disadvantageous in terms of a line speed as compared with the PON in which the whole interval of the access line is the optical fiber. As disclosed in JP-B-2007-516644 and JP-A-2007-124336, a method of improving a communication speed by binding a plurality of channels is also considered as one of measures for solving such a problem. However, even if four channels are bound, the line speed of up to about 160 Mbits/sec can be provided for the subscriber. On the other hand, the PON system using the standard of GPON (Gigabit PON) or GE-PON (Gigabit Ethernet PON) (Ethernet is a registered trademark) which is a main stream at present can provide the line speed of 1 Gbits/sec or higher for the subscriber.
It is considered that the most effective and simplest method of raising the speed of the access line of the cable Internet is a method whereby the station side apparatus is replaced by the OLT from the CMTS, the subscriber's apparatus is replaced by the ONU from the CM, and the access line between those apparatuses is replaced by the optical fiber and the optical splitter.
However, in such a case, it is necessary that each of the OLT and the ONU which are used for replacement has an interface which can be connected, without a problem, to a communicating apparatus which has already been used in its peripheral network.
That is, when mentioning about communication of a main signal, it is necessary that an interface of an upstream side network of the OLT (hereinbelow, called NNI (Network Node Interface) or SNI (Service Node Interface)) has compatibility with an interface of an upstream side network of the CMTS (hereinbelow, called NSI (Network Side Interface)) and can be connected to the router or layer 2 switch locating on the upstream side. It is also necessary that an interface of a downstream side network of the ONU (hereinbelow, called UNI (User Network Interface)) has compatibility with an interface of a downstream side network of the CM (hereinbelow, called CMCI (Cable Modem to CPE Interface)) and can be connected to a PC (Personal Computer) or a CPE (Customer Premise Equipment) such as a broadband router locating on the downstream side.
Further, when mentioning about communication of a control signal, it is necessary that, in place of the CMTS, the OLT receives a CMTS control signal for operating management, setting, and the like for the CMTS and reflects it to its own operation. In addition, it is necessary that the OLT relays a CM control signal which has been transmitted to the CM through the CMTS toward the ONU and the ONU receives the CM control signal and reflects it to its own operation.
Among those requirements, with respect to the main signal interface, since any of the NNI of the OLT and the NSI of the CMTS is substantially the same as the interface which the general router or layer 2 switch has, it can be also connected to peripheral communicating apparatuses without any problem. Also with respect to each of the UNI of the ONU and the CMCI of the CM, since it is a LAN (Local Area Network) interface which is used in a general home or enterprise, there is no problem in connecting performance.
However, with respect to the control signals, the control signals of the PON system and the control signals of the CMTS and the CM differ largely. The CMTS/CM control signals conform with the standard regarding the cable Internet which is generally called DOCSIS (Data-Over-Cable Service Interface Specifications) which is a trademark of Cable Television Laboratories, Inc. and has been specified by CableLabs (which is a trademark of Cable Television Laboratories, Inc.). The standard of those control signals differ from the standard of the OLT and the ONU. For example, in the DOCSIS standard, with respect to a data structure called MIB (Management Information Base) of a communicating apparatus management protocol called SNMP (Simple Network Management Protocol), it is specified that the CMTS and the CM should cope with such a data structure. However, a part of the data structure is unique to the DOCSIS standard and other communicating apparatuses including the OLT and the ONU do not cope with such a part of the data structure.
Particularly, with respect to the control of the CM, not only the control signal data structure such as an MIB of the SNMP but also a control method differ from those of the ONU. That is, according to the DOCSIS standard, the CM obtains an IP address of the CM itself from a DHCP server by using an IP address allocation protocol called DHCP (Dynamic Host Configuration Protocol). Address information of a TFTP (Trivial File Transfer Protocol) server, a file name of initial setting information which should be received from the TFTP server, and address information of a Time server are included in a message received from the DHCP server. Based on those information, the CM executes transmission of management information to an SNMP manager, obtainment of the initial setting information from the TFTP server, and IP communication for time synchronization with the Time server. Those various kinds of servers are generally located in a network of an upstream side of the CMTS.
The ONU does not have the IP address for control communication and does not execute the obtainment of the IP address which is executed by the DHCP. Therefore, the ONU itself does not execute communication of the SNMP, TFTP, and Time protocols as communication using an IP layer. In place of it, between the OLT and the ONU (hereinbelow, referred to as a PON interval), similar functions are realized by using a communication protocol based on the standard of the PON which is used by those apparatuses (or to which an expansion unique to a vendor has been added). For example, in the GE-PON, initial registration communication and time synchronous communication of the ONU to the OLT are made by an MPCP (Multi Point Control Protocol) protocol of the IEEE (Institute of Electrical and Electronics Engineers, Inc.) 802.3ah standard and communication of management information regarding the ONU is made by an OAM (Operation, Administration and Maintenance) sub-layer of the IEEE 802.3ah standard.
A method whereby while the difference between the PON and the CMTS/CM mentioned above is left, the PON is provided and operated is also considered. In such a case, however, since it is necessary that both of an apparatus management system for the CMTS/CM and an apparatus management system for the PON are provided for an upstream apparatus management network, the management becomes complicated. Further, although a method of integrating only a user interface for management of both of them is also considered, large development costs are required in such a case.
As another solving method, a method of changing the control communicating method of the PON interval to a method similar to that between the CMTS and the CM is also considered. In such a case, however, not only the control communication of the PON interval differs largely from that of the PON standard but also a development for making control communication similar to that between the CMTS and the CM is necessary for both of the OLT and the ONU.